1. Field of the Invention
The present invention relates to a receiving circuit of a communication equipment, and particularly relates to a receiving circuit in which superior stability in received signal strength can be obtained.
2. Description of the Related Art
Conventional receivers of communication equipments almost have a space diversity function using a plurality of antennas in order to reduce the influence of fading.
Conventionally, a receiving circuit in a space diversity path shown in FIG. 9 is known. The configuration and principle of the receiving circuit of a first conventional example will be described below with reference to FIG. 9.
In FIG. 9, an arriving radio wave induces a high-frequency signal in each of antennas 201 and 202. The induced signals pass through reception filters 203 and 204 respectively, and are supplied to first-stage reception amplifiers 205 and 206 so as to be amplified respectively. These outputs are supplied to frequency converters 207 and 208 so as to be subjected to frequency-reducing conversion, converted into intermediate-frequency band signals. These intermediate-frequency band signals are supplied to intermediate-frequency amplifiers 211 and 212 through filters 209 and 210 for eliminating unnecessary signals. The outputs of the amplifier 211 and 212 are supplied to detection circuits 215 and 216 through channel filters 213 and 214 for making only desired-frequency signals pass therethrough. The results of the detection circuits 215 and 216 are compared with each other in a comparison circuit 217, and a stronger signal in received signal strength is selected as an output 218. A diversity function is thus realized. A local frequency oscillator 219 supplies a local oscillation frequency signal to the frequency converters 207 and 208 in common.
That is, the above-mentioned first conventional example is constituted by two paths of receiving circuits which are independent of each other perfectly, and a common local signal circuit designed so as to make signal purity and phase uniform. An output in obtained by selection or combination of the two paths of received signals demodulated by these perfectly independent receiving circuits.
However, in the receiving circuits in the space diversity path in the above-mentioned first conventional example, not only the two independent receiving circuits consume power, but also their constituent parts increase the equipment volume. In addition, it is necessary to make the performance of the two paths of the receiving circuits equal to each other. Therefore, a second conventional example improving the above-mentioned first conventional example as shown in FIG. 10 has been known. FIG. 10 shows a spectrum spread communication diversity receiver disclosed in JP-A-7-87057, as the second conventional example. In FIG. 10, the spectrum spread communication diversity receiver is constituted by antennas 313 and 314, filters 315 and 316, amplifiers 317 and 318, a delay circuit 319, a combination circuit 320, a matched filter correlator 321, a phase detector 322, a delay circuit 323, a combination circuit 324, and a data demodulator 325.
The antenna 313 and the antenna 314 are made apart from each other by a distance of .lambda./3 or more, so that there is substantially no correlation between an SS received signal S5(t) from the antenna 313 side and an SS received signal S6(t) from the antenna 314 side. The filters 315 and 316 eliminate signals in any band other than the signals S5(t) and S6(t). The amplifiers 317 and 318 amplify the signals S5(t) and S6(t).
The delay circuit 319 delays an output S8(t) of the amplifier 318, while the delay time .tau. is set to .tau..gtoreq..tau.a in the condition that .tau..gtoreq.one chip length of PN code, and .tau.a designates the maximum delay time of a reflected wave which is influential on a direct wave. The combination circuit 320 combines an output S7(t) of the amplifier 317 and an output S9(t-.tau.) of the delay circuit 319. The combined SS received signal is separated in the time domain on the basis of the correlative operation with reference PN code performed in the correlator 321.
The reason why the delay is given by the delay circuit 319 is that the SS signal received on the antenna 313 side and the SS signal received on the antenna 314 side which are combined by the combination circuit 320 are separated in the time domain as correlative spike by the correlator 321 to thereby eliminate the interference between the SS signals of the antenna 313 side and of the antenna 314 side.
Then, in the SS signal received on the antenna 313 side, most of combined correlative spikes are suppressed when the difference in delay time between the direct wave and the reflected wave is within one chip length of PN code and the difference in phase between carriers in the correlative spikes outputted from the correlator for the respective received signals is 180.degree. (reverse phase). However, because the SS signal received on the antenna 314 side is not correlative with that of the antenna 313 side, a received signal with independent parameters are obtained. For example, if the above-mentioned difference in phase between carriers in the correlative spikes is 0.degree. (in-phase), the combined correlative spikes are hardly suppressed.
In such a state, if combination is performed under the condition of giving delay (by 4 chip length of PN code herein) to the SS signal received on the antenna 314 side, the correlative spikes are prevented frog being suppressed by multi-path, so that it is possible to improve the S/N ratio of the received SS signals, and it is possible to improve the performance of data demodulation.
In addition, in this conventional example, the correlative output of the correlator 321 is delayed and detected by the phase detector 322 in order to more improve the performance of data demodulation. Then, in the phase detector 322, detection is performed after multiplying a signal passing through the delay circuit with a delay of one data bit T with an original signal, and the multiplied signal is made to pass through a low pass filter so as to obtain an output.
This delay detection output divides the correlative output of a base band into two divisions. One of the two divisions is delayed in the delay circuit 323, and combined with the other in the combination circuit 324 again. Then, all the delay circuits are set to have an equal delay time. The output of the combination circuit 324 is supplied to the data demodulator 325. With this configuration, two SS signals S10(t) and S11(t-.tau.) received on the antennas 313 side and on the antenna 314 side respectively are suppressed by multi-path, and made into a signal S12 (t) by the combination of their correlative peaks, so that its correlative peak value in V.sub.1 +V.sub.2 when the peak values of the respective signals are V.sub.1 and V.sub.2 respectively. The S/N ratio is more improved, so that it is possible to improve the performance of data demodulation.
According to the above-mentioned second conventional example, the two paths of received signals are made into a single path in the combination circuit 320 and the following means, so that there is an effect to reduce the number of parts of the receiving circuits.
However, in the above-mentioned second conventional example, the canceling operation of carriers themselves between the two paths of received signals in left as it is, so that the output of the combination circuit 320, that is, the input signal of the matched-filter correlator 321 is attenuated with a high probability.
The aspect of this attenuation will be explained with reference to FIGS. 11A to 11D. FIGS. 11A to 11D show an example of a diversity effect in the second conventional example shown in FIG. 10. FIG. 11A designates a signal referred to as a so-called chip which is a product of a spread signal and an information signal of a spectrum spread communication signal sent to this receivers. FIG. 11B designates signals received by receiver antennas in the case of a modulation output in which a chip signal is modulated into 4 times of carrier frequency as a modulation wave by way of example. A portion (b-1) of FIG. 11B designates a signal received by an antenna 1, while a portion (b-2) of FIG. 11B a signal received by an antenna 2. Because the antenna 1 and the antenna 2 are made to be separated from each other by a distance of 1/3 or more of the wave length of a carrier in the second conventional example, the distance is net to one wave length herein, and the difference in phase .phi. is made 2.pi.. FIG. 11C designates the case where a signal path received from the antenna 1, that in, a signal of a branch 1, and a signal path received from the antenna 2, that is, a signal of a branch 2 are added to each other by a differential amplifier, that is, subtracted from each other. FIG. 11D designates the case where the signal of the branch 1 side is simply combined with the delayed signal of the branch 2 side in accordance with the second conventional example.
In FIGS. 11A to 11D, in order to make the explanation of the problem clear, it is assumed that there arises no canceling operation between arriving waves caused by multi-path fading in the antenna 1 and the antenna 2. It is understood from FIG. 11C that when the signals of the branches are added or subtracted simply, an adding/canceling operation of the signals similar to the canceling operation caused by multi-path fading arising on the antenna ends arises between the branches, so that there is a high probability that the time when the received signals disappear occurs frequently. In addition, it in understood from FIG. 11D that although there is some effect obtained by delaying a signal, the time of signal disappearance remains as it is with considerable frequency.
On the contrary, in the first conventional example, the signal superior in received conditions in selected from the branch 1 and the branch 2 every moment, or adaptive combination of signals is performed so as to eliminate the time when the received signals disappear. Therefore, it is impossible in the second conventional example to obtain a space diversity effect equal to that in the first conventional example.